1. Field of the Invention:
The present invention relates to an apparatus for controlling the rotational speed of an engine, and more particularly to an engine speed control apparatus for controlling the rotational speed of an engine when the engine operates under an excessive load condition.
2. Description of the Relevant Art:
There has been known an engine speed control process which effects PID (proportional plus integral plus derivative) control operations on a differential voltage corresponding to the difference between the actual rotational speed of an engine and a target or setpoint rotational speed, and adjusts the rate at which fuel is injected into the engine based on the output of the PID control, thereby controlling the rotational speed of the engine.
FIG. 6 of the accompanying drawings shows in block form a conventional engine speed control apparatus, which carries out the above known engine speed control process, in a fuel injection rate control system.
As shown in FIG. 6, the rotational speed of an engine 100 is detected by an engine speed detector which comprises a toothed rotor 102 mounted on the output shaft 101 of the engine 100, and a non-contact-type rotation sensor 103 for detecting the rotation of the toothed rotor 102. The rotation sensor 103 produces a pulse signal Pn whose period is proportional to the rotational speed of the engine 100. The pulse signal Pn is converted into a corresponding analog voltage en by a frequency-to-voltage (F/V) converter 104. The differential voltage .DELTA.e between the voltage en and a voltage es which corresponds to a target or setpoint rotational speed given by a target speed setting unit 105 is applied to a PID control circuit 106 in which the differential voltage .DELTA.e is subjected to proportional, integral, and derivative operations by a proportional amplifier 107, an integrator 108, and a differentiator 109, respectively. An output signal from the PID control circuit 106 is then delivered to a fuel injection unit 110, which adjusts the rate at which fuel is injected into the engine 100 thereby to control the rotational speed of the engine 100.
According to the illustrated engine speed control apparatus with the feedback loop, as the load on the engine 100 increases, the rotational speed thereof decreases, and therefore the fuel injection rate is increased in order to bring the rotational speed back to the target rotational speed. However, the prior engine speed control apparatus is incapable of coping with any engine loads in excess of the engine power output at the maximum fuel injection rate. When the engine is under such an overloaded condition, the rotational speed of the engine is reduced and so is the engine power output, sometimes resulting in an engine stall.
For example, it is assumed that an engine is mechanically coupled to a synchronous generator which generates electric energy having a commercial frequency. When a load such as a mercury lamp or an electric motor, which consumes a large current temporarily upon its starting, is connected to the output terminal of the synchronous generator, the load on the generator is temporarily increased, and hence the engine is overloaded when the connected mercury lamp or electric motor starts to be energized. The overloaded engine reduces the rotational speed and power output thereof, so that the electric energy generated by the generator mechanically coupled to the generator is also reduced in level. Regardless of the temporary overloaded condition, it takes the overall system a relatively long period of time to get back to its normal operation, and the output frequency of the generator is lowered until the normal operation is resumed.